Linear polyesters consisting essentially of residues of a bisphenol and dicarboxylic acids are important engineering thermoplastics especially noted for their excellent performance at high temperatures.
The transesterification reaction by a bisphenol and a diaryl ester of a dicarboxylic acid, preferentially in the presence of a small effective amount of a catalyst to prepare a thermoplastic linear polyester has been disclosed for example by British Pat. No. 924,697 assigned to Imperial Chemical Industries, Ltd. published Apr. 24, 1963.
The reaction, which is generally carried out in bulk, i.e. without solvent or diluent, with the reactants in molten condition generally employs temperatures in the range of above about 100.degree. to about 400.degree. or higher, e.g. above about 160.degree. to about 350.degree. C. for bisphenol A-isophthalate-terephthalate polyesters.
Since the reaction is reversible, provision is made for removal of the monohydroxy aromatic compound, e.g. phenol, displaced from the diaryl ester reactant during transesterification when a diphenyl ester is employed as the diaryl ester reactant, to cause the reaction to proceed to completion.
Monohydroxy aromatic compound removal is generally facilitated by gradually raising the temperature of the reaction mass from the minimum temperature required for onset of transesterification to about 300.degree.-400.degree. C. while gradually reducing the reaction pressure from about atmospheric pressure to about 0.1 mm or lower of mercury.
The viscosity of the polyester products is generally exceptionally high causing the viscosity of the reaction mass, as completion of the reaction is approached, to approach values above about 1.times.10.sup.6 to above about 10.times.10.sup.6 centipoises at reaction conditions, i.e. the range of viscosities of such polyesters as measured at shear rate of about 24 sec..sup.-1 and at about 320.degree. C.
Because of the above indicated wide variance in conditions, e.g. temperature and pressure, employed in the reaction and because of the necessity for wide variance in kind and degree of agitation, occasioned by the aforementioned viscosity increase during the reaction, the prior art has generally carried out the reaction in two stages, namely, a moderate temperature prepolymerization stage, to prepare a polyester oligomer prepolymer and a subsequent polymerization stage wherein said oligomer and the reactants are condensed to provide the polyester product.
The prepolymerization stage is carried out at about 100.degree. to about 350.degree. C. under moderate agitation conditions to prepare a low molecular weight polyester, i.e. prepolymer or oligomer, of relatively low molecular weight, corresponding to an intrinsic viscosity of less than about 0.1 to about 0.3 dl/g.
The polymerization stage is carried out at temperatures ranging to about 400.degree. C. employing extreme agitation conditions.
Because of difficulties in handling the aforementioned high viscosities of the molten reaction mass encountered in the transesterification reaction only batchwise mode or semi-continuous mode (wherein prepolymerization is batchwise and polymerization is continuous) have been suggested for preparation of these polyesters by transesterification.
Batchwise transesterification is in general not commercially attractive for production of large scale quantities of the polyester and generally provides an undesirably wide variance of product characteristics and properties from batch to batch of product.
In batchwise transesterification processes it is generally impossible to vary substantially the product molecular weight distribution, as measured by the product's polydispersity (which is further discussed below) by change of reaction conditions.
K. Eise et al., German Preliminary Application No. 2,232,877, published Jan. 17, 1974, disclose a semi-continuous transesterification polymerization of the polyester wherein the prepolymerization stage to prepare the oligomer (referred to as precondensate in the patent) is carried out batchwise and the subsequent polymerization stage is carried out continuously employing a vented extruder reactor, i.e. an extruder having provision for removal of vapors of the phenolic side product of the reaction.
Use of the extruder in accordance with the Eise et al. patent, generally has the disadvantage that the reaction mass in the extrusion reaction vessel is in the form of a thick mass, e.g. of greater than about 20 mm. thickness, principally adhering to the screw element of the extruder.
The residence time of the reaction mass in an extruder is relatively long compared to a thin film reactor giving rise to an undesirably long thermal history to the polyester. Such a long thermal history leads to a product of greater coloration which in the case of extreme product discoloration makes the product unacceptable for many uses. Under normal operations, the rate of new face exposure is limited, thus limiting the rate at which phenol can be eliminated from the system. As a result, relatively costly expenditures of energy are necessary to remove the phenolic side product to maintain a satisfactory rate of transesterification.
As an additional alternative to overcome the aforementioned difficulties in preparation of the polyester by transesterification especially the difficulties arising from the aforementioned high melt viscosity of the bisphenol-dicarboxylic acid polyester, the prior art (for example in U.S. Pat. No. 3,399,170, F. Blachke et al., assigned to Chemiche Werke Witten A.G., issued Aug. 27, 1968) has proposed modification of the structure of the polyester by introducing residues in minor proportion of alkylene and cycloalkylene glycols e.g. ethylene glycol, polybutylene glycol, into the polyester. Such structural modification of the polyester, while lowering the melt viscosity of the polyester thereby tending to overcome the aforementioned process difficulties, results in a modified polyester product which does not possess the high temperature performance characteristics of the unmodified bisphenoldicarboxylic acid polyesters of this invention.
According to technology developed by Teijin Co., Ltd., with respect to solid state polymerization, molecular weight build-up does occur in this method, but the method is limited to materials that do crystallize.